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1 <br /> ARCADIS GERAGHTY&MILLER <br /> indicates that the conditions in the depth interval of approximately 70 to 110 feet bgs at the Site <br /> ' are not conducive to significant constituent migration <br /> 6.5 INTRINSIC DEGRADATION OF SITE PETROLEUM HYDROCARBONS <br /> 6.51 Site Concentrations Declininq <br /> An examination of shallow wells at the Site with the longest monitoring record <br /> g g <br /> indicates that concentrations of petroleum hydrocarbon constituents have been declining over <br /> the 5-year period in winch monitoring has been conducted The shallow well with the highest <br /> recorded concentrations at the Site is monitoring well U-3 where TPHg was recorded at a <br /> concentration of 51,000 ug/L in August, 1994 Concentrations of TPHg in well MW-2 <br /> generally ranged in the vicinity of 40,000 to 50,000 ug/L at the beginning of the monitoring <br /> record TPHg concentrations in well U-3 declined to below 50,000 ug/L after mud-1997 <br /> TPHg concentration in U-3 was 28,000 ug/L in the February 5, 1999 monitoring event <br /> Similarly, benzene concentrations in well U-3 were recorded as high as 9,700 ug/L in 1990 and <br />' 9,100 in 1995 Benzene concentration in U-3 declined to below 5,000 ug/L in the May of <br /> 1996 and, with one exception in November, 1998, have remained there The February 4, 1999 <br />' benzene concentration in MW-2 was 4,300 ug/L <br />' The declunng concentrations over the period of the monitoring record provide primary <br /> evidence that intrinsic degradation ofpetroleum hydrocarbons is occurring at the Site <br />' 6.5.2 Stable Plume <br />' Further examination of the 5-year monitoring record indicates that the size of the <br /> shallow plume, as indicated by contouring of the Site benzene concentrations, is decreasing <br /> Figure 6-4 shows the interpreted nsoconcentration contour map of the distribution of benzene <br />' 41 <br />